Methods for treating a flue gas stream using a wet scrubber unit

ABSTRACT

Sorbent compositions, comprising a solid sorbent, a dispersive agent, and optionally a capture agent for enhanced wet-Flue Gas Desulfurization (wFGD) or wet scrubber unit function in a flue gas pollutant control stream is disclosed. The sorbent composition may include a sorbent with a dispersive agent, designed to enhance the dispersion of the sorbent in an aqueous sorption liquid of a wet scrubber unit, and therefore may be especially useful in EGU or industrial boiler flue gas streams that include one or more wet scrubber units. The sorbent composition may also include a capture agent useful in sequestering mercury and bromine, as well as other contaminants that may include arsenic, selenium and nitrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit as a continuationapplication of co-pending U.S. patent application Ser. No. 16/568,567,filed on Sep. 12, 2019, now U.S. patent Ser. No. ______, which is adivisional application of U.S. patent application Ser. No. 15/729,272filed on Oct. 10, 2017, now U.S. Pat. No. 10,421,037 which is adivisional application of U.S. patent application Ser. No. 14/696,409filed on Apr. 25, 2015, now U.S. Pat. No. 10,307,706, which claims thepriority benefit under 35 U.S.C. § 119(e) of U.S. ProvisionalApplication Ser. No. 61/984,165, filed on Apr. 25, 2014. Each of theseprior applications is incorporated herein by reference in its entirety.

FIELD

This disclosure relates to sorbent compositions having capability toenter, disperse and remain suspended in an aqueous sorption liquid of awet-Flue Gas Desulfurization (wFGD) unit (i.e., a wet scrubber unit) ofa flue gas stream emission control system and also relates tocontaminant removal from the wet scrubber unit.

BACKGROUND

Mercury (Hg) is a highly toxic compound and exposure at appreciablelevels can lead to adverse health effects for people of all ages,including harm to the brain, heart, kidneys, lungs, and immune system.Mercury is naturally occurring but is also emitted from various humanactivities, such as burning fossil fuels and other industrial processes.For example, in the United States about 40% of the mercury introducedinto the environment comes from coal-fired power plants.

In the United States and Canada, federal and state/provincialregulations have been implemented or are being considered to reducemercury emissions, particularly from coal-fired power plants, steelmills, cement kilns, waste incinerators and boilers, industrialcoal-fired boilers, and other coal combusting facilities. For example,the United States Environmental Protection Agency (U.S. EPA) haspromulgated Mercury Air Toxics Standards (MATS) which would, among otherthings, require coal-fired power plants to capture approximately 90% oftheir mercury emissions beginning in 2015.

The leading technology for mercury control from coal-fired power plantsis activated carbon injection (ACI). ACI involves the injection ofsorbents, particularly powdered activated carbon (PAC), into flue gasemitted by the boiler of a power plant. PAC is a porous carbonaceousmaterial having a high surface area, which exposes significant amountsof beneficial chemically functional and catalytic reaction sites,creating high adsorptive potential for many compounds, includingcapturing mercury from the flue gas.

ACI technology has shown the potential to control mercury emissions inmost coal-fired power plants, even those plants that may achieve somemercury control through control devices that are primarily designed forthe capture of other pollutants, such as wet or dry scrubbers used tocapture sulfur dioxide (SO₂) and other acid gases from the flue gasstream. Acid gases and acid gas precursors in the flue gas streamtypically come from three primary sources. The first is the coalfeedstock fed to the boiler. Certain types of coal inherently have highconcentrations of sulfur, nitrogen, chlorine, or other compounds whichcan form acid gases in the flue gas. For example, coals such as IllinoisBasin coal with high sulfur content (e.g., above about 0.5%) arebecoming more common as a boiler feedstock for economic reasons, as highsulfur coals tend to be cheaper than low sulfur coals. A second sourceis the selective catalytic reduction (SCR) step for controllingemissions of NON. An unintended consequence of this process is that SO₂in the flue gas can be oxidized to form SO₃. A third source is that thepower plant operator may inject SO₃ into the flue gas stream to enhancethe efficiency of the particulate removal devices, e.g., to avoidopacity issues and increase the effectiveness of an electrostaticprecipitator (ESP) in removing particulates from the flue gas stream.

Flue-gas desulfurization (FGD) is a set of technologies used to removesulfur dioxide from exhaust flue gases of coal-fired power plants. Oneof the common methods is to use a wet flue-gas desulfurization (wFGD)unit (i.e., a wet scrubber unit) that uses a slurry of a calcium-basedor sodium-based alkaline compound, usually limestone or lime. Contactingthe flue gas with the slurry scrubs (i.e., removes) contaminants fromthe flue gas, forming calcium sulfite or sodium sulfite. In FGD systemsutilizing forced oxidation, the corresponding sulfites are converted tosulfate byproducts that are collected while the scrubbed flue gas isemitted. The wet scrubber units have also been shown to be co-beneficialwith respect to the removal of mercury. The wet scrubber unit is oftenthe last emission control equipment before the stack, so it is criticalto ensure that mercury capture is achieved in or before the wet scrubberunit.

According to the U.S. EPA sixty-nine percent of coal-fired capacity willbe wet scrubbed by 2025, indicating a huge need for emission control inthe wet scrubber unit. Mercury control additives may also be utilized inwet scrubber units, and may include sulfur-based compounds that cantightly bond with mercury to form mercury sulfide. However, a largeamount of the additive (e.g., 1:1 to mercury) is typically required toachieve a high removal rate of mercury, adding to the cost of emissioncontrol. Additionally, sulfur-based additives only work well for mercurycapture in low oxidation-reduction potential (ORP) environments, or mayeven bring the ORP down, which can cause the acid gas capture in the wetscrubber unit to be less effective.

In addition to mercury emission, stringent regulations in regards toemission of other contaminants, such as selenium, arsenic, nitrates, andbromine, have been proposed. In a forced-oxidation wet scrubber unit,selenium exists as selenite (SeO₃ ²⁻) or selenate (SeO₄ ²⁻), dependingon the ORP in the aqueous phase. Selenate is more difficult to capturethan selenite, due to its higher water solubility. Halogens such asbromine, which may be in the form of a bromide compound such as calciumbromide, are often used to enhance mercury capture in the flue gasstream by adding the bromide compound to the combustible material (e.g.,coal) or directly to the flue gas stream, resulting in bromine build-upin the aqueous sorption liquid of the wet scrubber unit. Therefore, itmay be necessary to monitor bromine levels in the wet scrubber unit.Although bromine in oxidized forms, e.g., bromine (Br₂), bromate (BrO₃²⁻), etc. can be easily adsorbed by PAC, in low ORP conditions, thebromine exists as bromide (Br⁻) and therefore may not be readilycaptured by PAC. The wet scrubber unit serves as the last opportunity tocapture target emission compounds such as mercury, selenium, arsenic andbromine, as well as potentially arsenic and nitrates. Therefore,effective capture of these and other contaminants in the wet scrubberunit is crucial.

SUMMARY OF THE INVENTION

The use of activated carbon as a sorbent in wet scrubber units has beenproposed, but such use has been limited as activated carbon tends tostay at the air-water interface in the wet scrubber unit and creates afoam, preventing the activated carbon from dispersing well in theaqueous sorption liquid. As a result, the efficacy of the activatedcarbon for the removal and sequestration of contaminants such as mercuryfrom the aqueous sorption liquid is limited.

Other wet scrubber unit additives for mercury capture, such assulfur-based compounds, may be expensive and/or unsuccessful atsequestering mercury from the wet scrubber units. Further, sulfur-basedcompounds may limit the ability of the wet scrubber unit to capture acidgases, such as SO₂, by lowering the ORP. Prior art activated carbonsorbents may be ineffective at mercury capture within the wet scrubberunit due to their limited ability to disperse in the aqueous sorptionliquid contained in the wet scrubber unit due to the hydrophobic surfacecharacteristics of the solid sorbent.

It would be advantageous to provide a sorbent composition whichovercomes the limitations of conventional sorbents by modifying thesolid sorbent such that the solid sorbent can effectively enter into anddisperse within the aqueous sorption liquid of a wet scrubber unit andremain suspended therein to more efficiently capture and removecontaminants such as mercury, bromine/bromide, selenium, arsenic, and/ornitrate from the wet scrubber unit.

In one embodiment of the present disclosure, a sorbent composition forenhanced wet scrubber unit function is provided, i.e., for improvedperformance of a wFGD unit. The sorbent composition includes a solidsorbent and a dispersive agent, where the dispersive agent is selectedto cause the solid sorbent to more readily enter into and dispersewithin the aqueous sorption liquid contained in the wet scrubber unit.

A number of characterizations, refinements and additional features areapplicable to this embodiment. These characterizations, refinements andadditional features are applicable to this embodiment of a sorbentcomposition individually or in any combination.

In one characterization of the sorbent composition, the dispersive agentis selected from the group consisting of dispersants, deflocculants,surfactants, coupling agents and mixtures thereof. In one particularrefinement, the dispersive agent comprises a dispersant, and in afurther refinement the dispersive agent comprises a polymericdispersant. In another refinement, the dispersive agent comprises adeflocculant, and in yet a further refinement, the dispersive agent is aphosphate salt. In one particular refinement, the phosphate saltcomprises tri-sodium phosphate. In another refinement, the sorbentcomposition comprises at least about 0.05 wt. % of the tri-sodiumphosphate salt. In another refinement, the tri-sodium phosphate iscoated onto the solid sorbent.

In another refinement of the sorbent composition, the dispersive agentcomprises a surfactant. In yet another refinement, the dispersive agentcomprises a coupling agent.

In another characterization of the sorbent composition, the solidsorbent comprises a carbonaceous sorbent, such as powdered activatedcarbon (PAC).

In other characterizations of the sorbent composition, the sorbentcomposition comprises additional components. In one particularcharacterization, the sorbent composition further comprises a captureagent. In one refinement, the capture agent is selected from the groupconsisting of peroxides, persulfates, silver compounds and mixturesthereof. In one particular refinement, the capture agent comprisesammonium persulfate. In another particular refinement, the capture agentcomprises a silver compound, such as a silver salt. In anotherparticular refinement, the silver salt comprises silver nitrate. In yetanother refinement, the capture agent comprises a metal or ametal-containing compound, such as a metal or metal-containing compoundcomprising a metal that is selected from the group consisting ofcalcium, iron, and mixtures thereof. In one refinement of the captureagent, the capture agent is selected to capture and remove contaminantsfrom the flue gas stream and/or from the aqueous sorption liquid of thewet scrubber unit, the contaminants selected from the group consistingof mercury, arsenic, selenium, nitrate, and bromine.

In certain embodiments, the sorbent composition may also becharacterized by its capability to enter an aqueous sorption liquid of awet scrubber unit. In one refinement, the time needed for the sorbentcomposition to wet and enter an aqueous sorption liquid of a wetscrubber unit is not greater than about 50% of the time needed for theuntreated solid sorbent (i.e., a composition consisting essentially ofor consisting of the solid sorbent) to enter the aqueous sorptionliquid. In a further refinement, the time needed for the sorbentcomposition to wet and enter an aqueous sorption liquid of a wetscrubber unit is not greater than about 70% of the time needed for theuntreated solid sorbent to wet and enter the aqueous sorption liquid. Inyet a further refinement, the time needed for the sorbent composition towet and enter an aqueous sorption liquid of a wet scrubber unit is notgreater than about 90% of the time needed for the untreated solidsorbent to enter the aqueous sorption liquid.

In another embodiment of the present disclosure, a method for removingcontaminants from a flue gas stream is provided. In this embodiment, themethod comprises the steps of burning a combustible material, theburning creating a flue gas stream containing contaminants, adding asorbent composition to the flue gas stream, capturing the sorbentcomposition from the flue gas stream with an aqueous sorption liquidcontained in a wet scrubber unit, wherein the sorbent compositionsequesters contaminants from the aqueous sorption liquid. The sorbentcomposition may be the sorbent composition described above, includingany combination of the various refinements and additional features. Whenthe sorbent composition is captured by the aqueous sorption liquid inthe wet scrubber unit, the sorbent composition may advantageously enterthe aqueous sorption liquid (e.g., the sorbent composition is wetted bythe aqueous sorption liquid) and the sorbent composition may dispersewithin the aqueous sorption liquid to enhance the ability of the solidsorbent to capture and sequester contaminants from the aqueous sorptionliquid.

A number of characterizations, refinements and additional features areapplicable to this method of removing contaminants from a flue gasstream. These characterizations, refinements and additional features areapplicable to this embodiment of a method for removing contaminants froma flue gas stream, individually or in any combination.

In one characterization, the method comprises the further step of addinga second sorbent composition to the aqueous sorption liquid contained inthe wet scrubber unit. The second sorbent composition may be the samesorbent composition as the first sorbent composition, or may be adifferent sorbent composition, (e.g., having one or more differentcharacteristics.)

In another embodiment of the present disclosure, another method forremoving contaminants from a flue gas stream is disclosed. The methodincludes the steps of burning a combustible material to create a fluegas stream containing contaminants, adding a sorbent composition to anaqueous sorption liquid contained in a wet scrubber unit, wherein thesorbent composition enters the aqueous sorption liquid contained in thewet scrubber unit with substantially no foaming at the surface of theaqueous sorption liquid and wherein the sorbent composition disperseswell within the sorption liquid, and sequestering by the sorbentcomposition of contaminants from the aqueous sorption liquid. Thesorbent composition may be the sorbent composition described above,including any combination of the various refinements and additionalfeatures.

In another embodiment, a further method for removing contaminants from aflue gas stream is provided. The method includes the steps of burning acombustible material to create a flue gas stream containingcontaminants, adding a sorbent composition to a flue gas stream, andsequestering by the sorbent composition of contaminants. The sorbentcomposition may be the sorbent composition described above, includingany combination of the various refinements and additional features.

In yet another embodiment, a wet scrubber unit is disclosed. The wetscrubber unit is disposed in a flue gas train, and the wet scrubber unitcontains an aqueous sorption liquid, wherein the aqueous sorption liquidcomprises water, an alkaline compound dispersed in the water, a solidsorbent, and a dispersive agent, wherein the solid sorbent is dispersedthroughout the aqueous sorption liquid.

A number of characterizations, refinements and additional features areapplicable to this embodiment of a wet scrubber unit. Thesecharacterizations, refinements and additional features are applicable tothis embodiment of a method for removing contaminants from a flue gasstream, individually or in any combination.

In one characterization, the alkaline compound comprises limestone. Inanother characterization, the solid sorbent comprises a carbonaceoussorbent. In a particular refinement of this characterization, thecarbonaceous sorbent comprises activated carbon. In anothercharacterization, the dispersive agent is selected from the groupconsisting of dispersants, deflocculants, surfactants, wetting agents,coupling agents and mixtures thereof. In one particular refinement ofthis characterization, the dispersive agent comprises a deflocculant. Inanother refinement of this characterization, the dispersive agentcomprises a phosphate salt. In yet another refinement, the phosphatesalt comprises tri-sodium phosphate.

This embodiment of a wet scrubber unit may also include additionalfeatures. For example, in one characterization, the aqueous sorptionliquid also comprises a capture agent. For example, in onecharacterization the capture agent is selected from the group consistingof peroxides, persulfates, silver compounds and mixtures thereof. In oneparticular characterization, the capture agent comprises ammoniumpersulfate. In another particular characterization, the capture agentcomprises a silver compound, such as a silver salt. In one refinement ofthis characterization, the silver salt comprises silver nitrate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a flow sheet for the manufacture of a sorbentcomposition.

FIG. 2 schematically illustrates an exemplary plant configuration andmethod for the capture and sequestration of mercury from a flue gasstream.

FIG. 3 schematically illustrates an exemplary wet scrubber unitoperation.

FIG. 4 schematically illustrates mercury oxidation and capture in a wetscrubber unit.

FIG. 5A is a particle diameter graph illustrating the particle sizedistribution of comparative prior art Sample A measured as dry powder.

FIG. 5B is a particle diameter graph illustrating the particle sizedistribution of comparative prior art Sample A, measured in aqueousdispersion.

FIG. 5C is a particle diameter graph illustrating the particle sizedistribution of sorbent Sample C, measured in aqueous dispersion.

FIG. 6 schematically illustrates an exemplary open spray tower scrubberunit.

FIG. 7A is a graph illustrating amount of mercury associated with solidsin a scrubber as a function of time.

FIG. 7B is a graph illustrating amount of mercury associated with solidsin a scrubber as a function of time.

FIG. 8 is a graph illustrating mercury emission from a stack in abeta-test conducted over several months.

DETAILED DESCRIPTION

While the use of solid sorbents such as activated carbon in a wetscrubber unit (e.g., in the aqueous sorption liquid of a wet scrubberunit) has been proposed, the use of solid sorbents in this manner hasmet with limited success. It is believed that this limited success isdue, at least in part, to the foaming that commonly occurs at theinterface between the liquid and the air when the solid sorbent is addedto the wet scrubber liquid. It is believed that this foaming may hinderthe ability of the solid sorbent to enter, disperse, and remainsuspended in the aqueous sorption liquid (e.g., in a lime slurry) thatis used to capture SO₂ in the wet scrubber unit. It is believed that theinability of the solid sorbent to enter, disperse and remain suspendedin the aqueous sorption liquid inhibits the ability of the solid sorbentto effectively remove contaminants from the wet scrubber unit such asmercury (Hg).

It would be advantageous to provide a sorbent composition whichmitigates some of the limitations of conventional sorbents when used ina wet scrubber unit. According to the present disclosure, suchlimitations may be mitigated by modifying a solid sorbent such that thesolid sorbent can effectively enter into and disperse within the aqueoussorption liquid and remain suspended therein to more efficiently captureand remove contaminants such as mercury, bromine/bromide, selenium,arsenic, and/or nitrate from the wet scrubber unit.

According to one embodiment, to facilitate contaminant removal by thesolid sorbent in an aqueous sorption liquid, the sorbent composition maycomprise a solid sorbent and a dispersive agent. The solid sorbent maybe any material that may adsorb, absorb or otherwise attract moleculesto its surface typically due to the sorbent having a high surface area.Solid sorbents may include aluminum oxides, silica, polypropylenefibers, cellulosic fibers, zeolite, molecular sieves, and/orcarbonaceous sorbents. In one characterization, the solid sorbentcomprises a carbonaceous sorbent such as an activated carbon. Powderedactivated carbon (PAC) may be particularly effective as the solidsorbent.

Various embodiments of a sorbent composition are provided that areparticularly useful when placed (e.g., injected) into a wet scrubberunit (e.g., into the aqueous sorption liquid of a wet scrubber unit)that is used in a flue gas treatment train to treat a flue gas stream(e.g., from a coal-burning boiler or a waste to energy boiler) torapidly and efficiently capture and remove contaminants, such asmercury, from the flue gas stream. The sorbent composition may be addedto the aqueous sorption liquid by adding the sorbent composition to theflue gas stream (i.e., contacting the sorbent composition with the fluegas stream) at a location upstream of the wet scrubber unit, and/or maybe added to the wet scrubber unit independently, i.e., independent fromthe flue gas stream. In this regard, the sorbent composition mayadvantageously be wetted by and enter the aqueous sorption liquid in thewet scrubber unit to recover sufficient amounts of mercury to meetmercury removal criteria, such as applicable government regulations, andto possibly remove and sequester other contaminants from the aqueoussorption liquid.

In some embodiments, the solid sorbent may have various beneficialphysical attributes such as a relatively small particle size, a highsurface area and a well-controlled pore size distribution. Suchcharacteristics may be particularly advantageous when the sorbentcomposition is injected into the flue gas stream before the flue gasstream enters the wet scrubber unit.

For example, the solid sorbent may have a median particle size of notgreater than about 100 micron, such as not greater than about 75 micron,such as not greater than about 50 micron, such as not greater than about30 micron, or even not greater than about 15 micron. Characterized inanother way, the median particle size may be at least about 5 micron,such as at least about 6 micron or even at least about 8 micron. The D50median particle size may be measured using techniques such as lightscattering techniques (e.g., using a Saturn DigiSizer II, available fromMicromeritics Instrument Corporation, Norcross, Ga.). A relatively smallmedian particle size, such as not greater than about 15 micron, meansgreater surface area per volume of the solid sorbent. The increasedsurface area may result in many benefits, including, but not limited to,increased exposure of the mercury to chemical species (e.g., elements orcompounds) disposed on the solid sorbent surface, increased areaavailable for reactions to occur, and thus overall improved reactionkinetics, particularly when the sorbent composition is entrained in theflue gas stream before entering the wet scrubber unit.

In another characterization, the solid sorbent has a relatively highsurface area. For example, the solid sorbent may have a surface area ofat least about 350 m²/g, such as at least about 400 m²/g or even atleast about 500 m²/g. Surface area may be calculated using theBrunauer-Emmett-Teller (BET) theory that models the physical adsorptionof a monolayer of nitrogen gas molecules on a solid surface and servesas the basis for an analysis technique for the measurement of thespecific surface area of a material. BET surface area may be measuredusing the Micromeritics TriStar II 3020 (Micromeritics InstrumentCorporation, Norcross, Ga.).

In one example, a solid sorbent comprises PAC with a median particlesize or D50 of about 25 micron, and is measured to have a surface areain the range of from about 480 to about 600 m²/g. In another example, asolid sorbent comprises PAC with a D50 of about 10 micron, and ismeasured to have a surface area in the range of from about 440 to about570 m²/g.

The solid sorbent may also have a high pore volume and a well-controlleddistribution of pores, particularly among the mesopores (i.e., from 20 Åto 500 Å width) and the micropores (i.e., not greater than 20 Å width).A well-controlled distribution of micropores and mesopores is desirablefor effective removal of mercury from the flue gas stream. While notwishing to be bound by any theory, it is believed that the mesopores arethe predominant structures for capture and transport of the oxidizedmercury species to the micropores, whereas micropores are thepredominate structures for sequestration of the oxidized mercuryspecies.

In this regard, the sum of micropore volume plus mesopore volume of thesolid sorbent may be at least about 0.10 cc/g, such as at least 0.20cc/g, and at least about 0.25 cc/g or even at least about 0.30 cc/g. Themicropore volume of the solid sorbent may be at least about 0.10 cc/g,such as at least about 0.15 cc/g. Further, the mesopore volume of thesolid sorbent may be at least about 0.10 cc/g, such as at least about0.15 cc/g. In one characterization, the ratio of micropore volume tomesopore volume may be at least about 0.7, such as 0.9, and may be notgreater than about 1.5. Such levels of micropore volume relative tomesopore volume advantageously enable efficient capture andsequestration of oxidized mercury species by the solid sorbent. Porevolumes may be measured using gas adsorption techniques (e.g., N₂adsorption) using instruments such as a TriStar II Surface Area Analyzer(Micromeritics Instruments Corporation, Norcross, Ga., USA).

The solid sorbent may comprise a porous carbonaceous sorbent material(e.g., fixed carbon) that is adapted to provide a large surface area inthe appropriate pore size to sequester contaminants from the aqueoussorption liquid such as mercury, bromine, selenium, etc. For example,the solid sorbent may include at least about 10 weight percent fixedcarbon, such as at least about 15 weight percent or even at least about20 weight percent fixed carbon. In some characterizations, the fixedcarbon content of the solid sorbent may not exceed about 95 weightpercent, such as not greater than about 85 weight percent, such as notgreater than about 75 weight percent, such as not greater than about 60weight percent, or such as not greater than about 55 weight percentfixed carbon. Due to a well-controlled pore structure and the presenceof the other components in the solid sorbent, a relatively low amount offixed carbon may be required for sequestration of contaminants.

Another component of the solid sorbent may be in the form of minerals.In one characterization, such minerals may be native to the feedstockfrom which the carbonaceous solid sorbent is formed (e.g., byactivation). In another characterization, some portion of the mineralsmay be separately added to the solid sorbent, which requires additionalexpense. While not wishing to be bound by any theory, it is believedthat such minerals may advantageously facilitate the oxidation of theelemental mercury in the flue gas stream if the solid sorbent isinjected into the flue gas stream ahead of the wet scrubber unit. Thepresence of such minerals may thereby enhance the kinetics of themercury oxidation such that a reduced contact time with the flue gasstream is required to oxidize and remove mercury from the flue gasstream as compared to sorbent compositions without such minerals.

The minerals may advantageously be selected from minerals including, butnot limited to, aluminum-containing minerals, calcium-containingminerals, iron-containing minerals, silicon-containing minerals (e.g.,silicates), sodium-containing minerals, potassium-containing minerals,zinc-containing minerals, tin-containing minerals, magnesium-containingminerals, and combinations thereof. The minerals may predominantly beoxide-based minerals, such as metal oxide minerals (e.g., CaO, Fe₂O₃,Fe₃O₄, FeO, Al₂O₃), and silicates (e.g., Al₂SiO₅). In onecharacterization, the minerals predominantly include metal oxides,particularly aluminum oxides and iron oxides. In anothercharacterization, the minerals include calcium-containing minerals,iron-containing minerals and/or aluminosilicates. These types ofminerals are well adapted to catalyze the oxidation reaction of themercury. Iron-containing minerals are particularly well adapted tocatalyze the oxidation reaction, and in one characterization, theminerals include at least 1 wt. % iron-containing minerals. The mineralsmay be intimately intertwined within the sorbent composition within awell-controlled porous structure that facilitates the oxidation, captureand removal of mercury. Such minerals, particularly iron-containingminerals and calcium containing minerals, may also facilitate thesequestration of contaminants (e.g., selenium) in the wet scrubber unit.The presence of elemental iron and/or elemental calcium (e.g., metalliciron and/or calcium) may also facilitate the sequestration ofcontaminants such as selenium and arsenic in the wet scrubber unit.

In one characterization, the solid sorbent may include at least about 20weight percent (wt. %) of the minerals, such as at least about 25 wt. %and even at least about 30 wt. % of the minerals. However, excessiveamounts of the minerals may be detrimental to the capture of mercury. Inthis regard, the solid sorbent may include not greater than about 50 wt.% of the minerals, such as not greater than about 45 wt. % of theminerals. Advantageously, the solid sorbent may include not greater thanabout 40 wt. % of the minerals, such as not greater than about 35 wt. %minerals. The total mineral content may be measured by a TGA701Thermalgravitmetric Analyzer (LECO Corporation, St. Joseph, Mich.). Thespecific types and amount of particular minerals may be measured by theNiton XL3t X-Ray Fluorescence (XRF) Analyzer (Thermo Fisher ScientificInc., Waltham, Mass.).

In accordance with the present disclosure, the sorbent compositionincludes a solid sorbent, e.g., as described above, and also includes adispersive agent, e.g., an effective amount of a dispersive agent toincrease the wettability and/or the dispersion of the solid sorbent inthe aqueous sorption liquid. The sorbent composition may include othercomponents that may be added to the sorbent composition and/or that maybe integrally formed when producing the solid sorbent.

The sorbent composition may have additional beneficial surface chemistrycharacteristics, particularly when the sorbent composition is contactedwith the flue gas before contacting the wet scrubber unit. For example,flue gas is typically acidic. When sequestering metals such as mercuryin an acidic environment, it is advantageous to solubilize the metalprior to capture, which often requires specific ionic conditions. Inthis regard, the sorbent composition may have a basic surface pH ofgreater than about 8 and not greater than about 12. Such basic surfacechemistry may be inherent to the solid sorbent (e.g., a result of themanufacturing process), or the sorbent composition may comprise anadditive in the form of a base that is capable of neutralizing acidiccomponents in the flue gas.

Moreover, the sorbent composition may also include an amount ofaqueous-based solubilizing medium such as water, particularly when thesorbent composition is contacted with the flue gas before contacting thewet scrubber unit. An aqueous-based solubilizing medium can combat theacidity of the flue gas, enhance the mass diffusional kinetics ofmercury oxidation and sequestration reactions by solubilizing oxidizedmercury species within the pore structure, and prevent captured mercuryfrom re-solubilizing and reentering the flue gas. In this regard, thesorbent composition may include at least about 2 wt. % of thesolubilizing medium, such as at least about 3 wt. % or at least about 6wt. % of the aqueous-based solubilizing medium. However, the amount ofsolubilizing medium in the sorbent composition should be not greaterthan about 12 wt. %, such as not greater than about 10 wt. % to avoidinterfering with the mercury oxidation reaction(s) and consuming carbonadsorption capacity. The solubilizing medium may be added to the sorbentcomposition, but typically will be formed naturally during manufactureand/or storage and transport of the solid sorbent and/or the sorbentcompositions.

The enhanced reaction and diffusional kinetics of the sorbentcomposition may enable coal-burning facilities (e.g., a coal-fired powerplant) to rapidly and efficiently capture mercury to meet regulatorylimits on mercury emissions. For example, the U.S. EPA MATS set mercuryemission limits based on the amount of mercury per amount of powerproduced of 1.2 lb. Hg/Tbtu for coal-fired power plants combusting highrank coals (i.e., coals having >8,300 Btu/lb.) and of 4.0 lb. Hg/Tbtufor facilities combusting low rank coals (i.e., coals having <=8,300Btu/lb.).

FIG. 1 is a flow sheet that illustrates an exemplary method for themanufacture of a sorbent composition in accordance with one embodiment,i.e., for the manufacture of a carbonaceous solid sorbent such asactivated carbon and the addition of other components to form thesorbent composition. The manufacturing process may begin with acarbonaceous feedstock 101 such as low-rank lignite coal with arelatively high content of natural deposits of native minerals. In themanufacturing process, the feedstock 101 is subjected to an elevatedtemperature and one or more oxidizing gases under exothermic conditionsfor a period of time to increase surface area, create porosity, altersurface chemistry, and expose and exfoliate native minerals previouslycontained within feedstock. The specific steps in the process include:(1) dehydration 102, where the feedstock is heated to remove the freeand bound water, typically occurring at temperatures ranging from100-150° C.; (2) devolatilization 103, where free and weakly boundvolatile organic constituents are removed, typically occurring attemperatures above 150° C.; (3) carbonization 104, where non-carbonelements continue to be removed and elemental (fixed) carbon isconcentrated and transformed into random amorphous structures, typicallyoccurring at temperatures around 350-800° C.; and (4) activation 105,where steam, air or another oxidizing agent is added and pores aredeveloped, typically occurring at temperatures above 800° C. Themanufacturing process may be carried out, for example, in a multi-hearthor rotary furnace. The manufacturing process is not discrete and stepscan overlap and use various temperatures, gases and residence timeswithin the ranges of each step to promote desired surface chemistry andphysical characteristics of the manufactured product.

After activation 105, the thermally treated product (e.g., granularactivated carbon) may be subjected to a comminution step 106 to reducethe particle size (e.g., the median particle size) of the product (e.g.,to form powdered activated carbon). Comminution 106 may occur, forexample, in a mill such as a roll mill, jet mill or other like device.Optionally, comminution 106 may be carried out for a time sufficient toreduce the median particle size of the thermally treated product, e.g.,to not greater than about 100 micron, such as not greater than about 75micron, such as not greater than about 50 micron, such as not greaterthan about 30 micron or even not greater than about 15 micron.

Advantageously, the sorbent composition may have a relatively highHardgrove Grindability Index (HGI), as measured by ASTM Method D409. TheHGI was developed to empirically measure the relative difficulty ofgrinding coal to the particle size necessary for complete combustion ina coal boiler furnace. The use of HGI has been extended to grinding coalfor other purposes such as iron-making, cement manufacture and chemicalindustries utilizing coal. Particulate materials of low value HGI aremore difficult to grind than those with high values. Mill capacity alsofalls when grinding materials with a lower HGI. In this regard, the HGIof the sorbent composition may be at least about 80 such as at leastabout 90, at least about 100 or even at least about 110. The relativelyhigh HGI enables the average particle size to be reduced with relativelylow energy consumption. Further, the relatively soft materials of thesorbent composition will lead to reduced erosion (e.g., attrition) ofthe comminuting equipment as compared to harder materials. While notwishing to be bound by any theory, it has been observed that utilizinglignite matter (e.g., lignite coal) feedstock will lead to a relativelyhigh HGI.

In the event that manufacturing conditions result in a greater number ofcarbonaceous particles that have a very fine size than is desired,classification 107 may be carried out to remove such very fine particlesfrom the larger carbonaceous particles. For example, classification 107may be carried out using an air classifier, screen/mesh classification(e.g., vibrating screens) or centrifugation. Smaller particles may alsobe agglomerated to reduce the concentration of fine particles. Thepotential benefits of such a classification step are described in U.S.patent application Ser. No. 14/201,398 by McMurray et al., which isincorporated herein by reference in its entirety.

In accordance with the present disclosure, one or more dispersiveagent(s) is included in the sorbent composition to improve thecontaminant capture efficiency of the solid sorbent when introduced tothe aqueous sorption liquid of the wet scrubber unit. In onecharacterization, the one or more dispersive agent(s) is provided as acomponent of the sorbent composition to increase the hydrophiliccharacter of the solid sorbent surface and allow the sorbent compositionto enter, disperse and remain suspended within the aqueous sorptionliquid of a wet scrubber unit more easily than a solid sorbent lackingsuch a dispersive agent(s). Further, the dispersive agent(s) may reducefoaming at the interface of the liquid-air layer in the wet scrubberunit. The dispersive agent(s) may be any material that associates withthe solid sorbent surface to enhance its hydrophilic character (e.g., todecrease its hydrophobic character) and enables better dispersion of thesolid sorbent in the aqueous environment. Dispersive agents may include,as non-limiting examples, materials commonly referred to as dispersants,including polymeric dispersants, deflocculants, surfactants (e.g.,wetting agents), emulsifying agents or emulsifiers, and/or couplingagents.

For example, the dispersive agent may be a dispersant i.e., a compoundthat improves the stability of particles in a liquid suspension byreducing the tendency of the solid sorbent particles to agglomerate andsettle from the bulk of the sorption liquid. The dispersant compound maybe a non-surface active compound or a surface active compound. Phalateesters such as polyvinyl chloride (PVC), polycarboxylate ether (PCE), orpolycarboxylate (PC) may be utilized as dispersants. Useful dispersantsmay also include sulfonated naphthalene or sulfonated melamine. Otheruseful dispersants may include benzoates, terephthalates, esters,vegetable oils, sulfonamides, organophosphates, glycols/polyethers, andpolybutene.

The dispersant may also be a compound that is commonly categorized as anemulsifier. Emulsifiers are substances that stabilize an emulsion byincreasing kinetic stability of the mixture. Common examples ofemulsifiers that may function as a dispersant for the solid sorbentsinclude lecithin, mucilage, sodium stearoyl lactylate, diacetyl tartaricacid ester of monoglyeride (DATEM). Detergents may also be useful as adispersant. Other examples of useful emulsifiers include emulsifyingwax, certearyl alcohol, polysorbate 20 and ceteareth 20.

Polymeric dispersants are characterized as higher molecular weightpolymer compounds comprised of polymer segments belonging to such groupsas, but not limited to, polyurethanes, polyacrylates, polyethers,polyesters, and copolymers thereof. Polymeric dispersants may be eithernon-ionic or ionic in character, and function to stabilize particles ina liquid by steric stabilization, or by a combination of chargestabilization and steric stabilization. Polymeric dispersants may offerbenefits such as versatility, thermal stability, electrolyte stability,stability under high shear conditions, and higher performance, even witha reduced concentration of dispersant.

The dispersive agent may also be a deflocculant. Deflocculants arechemical additives that reduce the settling of particles from asuspension. Deflocculants are typically low-molecular weight anionicpolymers that neutralize positive charges on suspended particles, suchas aryl-alkyl derivatives of sulfonic acids including polyphosphates,lignosulfonates, tannins, or water-soluble synthetic polymers. Examplesof particular deflocculants that may be useful as a dispersive agentinclude polyphosphates, such as a sodium polyphosphates, such examplesbeing tri-sodium phosphate (TSP, Na₃PO₄), or di-ammonium hydrogenphosphate ((NR₄)₂HPO₄), or sodium hexametaphosphate (Na₆P₆O₁₈).

The dispersive agent may also be a surfactant. Surfactants, which arealso known as wetting agents, lower the surface tension between theliquid and the solid, as in the case of a solid sorbent entering theaqueous phase of a wet scrubber unit. Surfactants may also reduce thesurface tension at liquid-air interfaces, reducing the generation offoam. Surfactants may be categorized as detergents, emulsifiers, ordefoamers. In general surfactants act to form a hydrophilic surface onan otherwise hydrophobic compound or particle. Examples include linearalkylbenzenesulfonates, lignin sulfonates, fatty alcohol ethoxylates,and alkylphenol ethoxylates. Surfactants may be fluorosurfactants,siloxane surfactants, and surfactants with polyether chains such aspolyethylene and polypropylene oxides. Anionic surfactants may includesulfate, sulfonates, phosphates, and carboxylates, such as ammoniumlauryl sulfate, or sodium lauryl sulfate, sodium stearate, sodiumlauroyl sarcosinate, and caboxylated fluorosurfactants such asperfluorononanoate, or perfluorooctanoate. A surfactant may have acationic head group such as pH-dependent primary, secondary, or tertiaryamines or permanently charged quaternary ammonium cations such as cetyltrimethylammonium bromide. Further, zwitterionic or amphotericsurfactants have both cationic and anionic centers, the cationic partprimarily based on primary, secondary, or tertiary amines, and theanionic part being more variable but often including sulfonates. Usefulnon-ionic surfactants may include fatty alcohols (e.g., ethoxylatedfatty alcohols), cetyl alcohol, stearyl alcohols, cetostearyl alcohol,and oleyl alcohol.

The dispersive agent may also comprise a coupling agent. The couplingagents may act as a bridge that links a particle to the aqueous phasesuch as through covalent bonding, and/or through secondary interactionssuch as hydrogen bonding or ionic bonding. Examples of such hydrophiliccoupling agents include epoxy, amine, hydroxyl, isocyanate, acrylic,mercapto, and phosphine functionalized silane and siloxane compounds.

In one particular embodiment, the dispersive agent comprises apolyphosphate, e.g., a phosphate salt. Phosphate salts function byreducing the charge density around agglomerates, creating smallerparticulates that can be more readily dispersed in the aqueous sorptionliquid. Phosphate salts may be particularly advantageous becausephosphate salts tend to wet quickly, disperse well and are relativelyinexpensive. Particularly useful phosphate salts include tri-sodiumphosphate (TSP), di-ammonium hydrogenphosphate, and sodiumhexametaphosphate

Referring back to FIG. 1, one or more dispersive agents may be contactedwith the activated carbon. For example, the dispersive agent(s) be insolution form may be contacted with the sorbent prior to 106A or after106B comminution 106. In this regard, the dispersive agent may be coatedon or impregnated in the solid sorbent prior to 106A or after 106Bcomminution 106, or may be admixed with the sorbent as dry agents eitherprior to 106A or after 106B comminution 106. For example, a dispersiveagent (e.g., TSP), being for example solubilized in an aqueous solution,may be sprayed onto a sorbent such as PAC to form a sorbent compositioncomprising at least about 0.05 wt. % of the dispersive agent, or even atleast 0.1 wt. % or greater of the dispersive agent. In onecharacterization, the sorbent composition comprises not greater thanabout 50 wt. % of the dispersive agent, such as not greater than about20 wt. %, not greater than about 15 wt. % or even not greater than about10 wt. % of the dispersive agent. It will be appreciated that the amountof dispersive agent should be sufficient to enable dispersion of thesolid sorbent into the aqueous sorption liquid. As such, the sorbentcomposition may show an increased ability to enter and disperse withinan aqueous sorption liquid within a wet scrubber unit as compared tountreated solid sorbents. For example, the wetting time of a sorbentcomposition including a dispersive agent as disclosed herein may be atleast about 70%, or even at least about 74%, or even at least about 85%faster than the wetting time of the same solid sorbent that is nottreated with a dispersive agent.

The sorbent composition disclosed herein may include other additives inaddition to the dispersive agent. In one embodiment, the sorbentcomposition may include one or more of a capture agent(s) to facilitatethe capture of contaminants from the aqueous sorption liquid in the wetscrubber unit. The capture agent(s) may be selected from an oxidant,chelating or bonding agent, precipitant, or metal species thatfacilitates the removal of mercury, bromide, selenium, arsenic, and/ornitrate from the aqueous sorption liquid within the wet scrubber unit.The capture agent may be used in combination with the dispersive agent.

Examples of useful capture agents that are categorized as oxidants(e.g., oxidative capture agents) include persulfates, such as ammoniumpersulfate or sodium persulfate, or peroxides such as hydrogen peroxideor potassium peroxymonosulfate. As such, ammonium persulfate (APS,(NH₄)₂S₂O₈) may be added to the aqueous sorption liquid of the wetscrubber unit such that bromide (Br) is oxidized to the larger bromateanion (BrO₃ ⁻), which increases its sorbent capture potential accordingto the following reaction:

Br⁻+3(NH₄)₂S₂O₈+3H₂O→BrO₃ ⁻+6NH₄HSO₄.

The capture agent may also be characterized as a chelating or bondingagent. Examples of useful chelating or bonding agents may include flyash, inorganic sulfur compounds, chitosan and chitosan derivatives,crown ethers, cyclodextrins, ethylenediamine tetraacetic acid (EDTA),polymeric materials containing amine groups, iron compounds, sulfurcompounds, and alumina.

The capture agent may also be a precipitant. Precipitants that may beuseful as a capture agent include ionic compounds such as silver ionsthat have the ability to form cations in solution (e.g., in the aqueoussorption liquid) to bind the charged contaminant particles. For example,silver nitrate (AgNO₃) may form Ag⁺ and NO₃ ⁻ to bind to the negativelycharged bromide ion (Br) to form AgBr which, due to its low solubilityin water, may form a precipitate that can be readily separated from theaqueous sorption liquid of the wet scrubber unit either by deposition onthe surface of the solid sorbent or as a self-contained solid.

The capture agent may also be a metal species. Metal species that may beuseful as a capture agent include calcium or iron, and compoundscontaining calcium or iron may be utilized that are capable of reactingwith Se and As, either in the zero valent form or as an oxidized speciessuch as selenium oxide (SeO₂), selenite (SeO₃ ²⁻) or selenate (SeO₄ ²⁻),to complex with and remove Se from either the flue gas or from theaqueous sorption liquid of the wet scrubber unit, via adsorption by thesorbent or by precipitation.

In a manner similar to the dispersive agent(s), the capture agent(s) maybe added in to the sorbent in the form of a solution (e.g., sprayed onor slurried with the sorbent) to coat and/or impregnate the solidsorbent with the capture agent prior to 106A or after 106B comminution106, or may be admixed with the sorbent as dry agents either prior to106A or after 106B comminution 106. The capture agent(s) may optionallyassociate with the solid sorbent of the sorbent composition either priorto, or following, the capture of one or more contaminant species. As anon-limiting example, a silver nitrate solution may be sprayed ontogranular activated carbon (GAC) before comminution 106A, or alternatelyonto the PAC that is created by the comminution step 106, as in step106B. Alternately, substantially dry silver nitrate may be dry mixedwith GAC and milled to form a PAC-silver nitrate mixture of appropriateparticle size. Further, the dry silver nitrate may be ground to anappropriate size and admixed with PAC after comminution as in step 106B.

FIG. 2 schematically illustrates a sample flue gas stream emissioncontrol system (i.e., a flue gas treatment train) that may be utilizedat an electricity generating unit (EGU) or industrial boiler site.Components of the system may include a bag house (i.e., a fabricfilter), electrostatic precipitator (ESP), an air heater (AH), wet fluegas desulfurization (wFGD) or wet scrubber unit, and/or a selectivecatalytic reactor (SCR). The plant's boiler 201 produces a flue gasstream 202. The flue gas stream may flow through a through an SCR 203,hot-side ESP 204, AH 205, bag house (BH) 206, cold side ESP 207, thenthrough a wet-Flue Gas Desulfurization (wFGD)/SO₂ scrubber, also calleda wet scrubber unit 208, and then out through the stack 209. PAC may beinjected at various points to be either entrained in the flue gas stream210, e.g., added upstream 211A of and/or directly to 211B the wetscrubber unit.

It should be understood that various operations are possible and thatthe sorbent compositions disclosed herein are particularly well suitedfor use in flue gas treatment systems with a wet scrubber unit 208, inthat the sorbent compositions may have increased ability to enter anaqueous sorption liquid in the wet scrubber unit along with othercharacteristics favorable to mercury, selenium, arsenic, nitrate, andbromide contact, capture, and/or conversion which aid in contaminantremoval from the aqueous sorption liquid. A sorbent composition that isspecially designed to enter the aqueous sorption liquid in the wetscrubber unit may be added to a flue gas stream at point 211A, i.e.,before the flue gas enters the wet scrubber unit 208. Alternately, thesorbent composition may be added directly to the wet scrubber unit 208,such as at point 211B. Methods for use of a sorbent in a wet scrubberunit have been described before such as in U.S. Pat. No. 7,722,843 toSrinivasachar and U.S. Pat. No. 7,727,307 to Winkler, each of which isincorporated herein by reference in its entirety.

Although the additives disclosed herein (e.g., the dispersive agent, thecapture agent, or other additives) may be combined with the solidsorbent prior to contacting the sorbent composition with the aqueoussorption liquid, e.g., by admixing with or coating the additives with oronto the solid sorbent, the additives may also be added to the aqueoussorption liquid separate from the sorbent. That is, a solid sorbent(e.g., activated carbon) may be added directly to the aqueous sorptionliquid in the wet scrubber unit while other additives, such as thedispersive agent, are separately added to the aqueous sorption liquid.In one characterization, the dispersive agent is coated onto the solidsorbent and another agent, such as a capture agent, is added separatelyto the aqueous sorption liquid.

FIG. 3 illustrates processes related to the operation of a wet scrubberunit 208. Limestone 301, being calcium carbonate, is mixed with water302 and is ground in a tank 303, to form an aqueous sorption liquid(e.g., a slurry) that enters a limestone slurry tank 304 that feeds thewet scrubber unit 208. Compressed air 305 is fed up through the bottomof the wet scrubber unit 208. Solid-liquid slurry (e.g., waste sorptionliquid or effluent) is removed via pump 306 from the bottom of the wetscrubber unit 208 and filtered, and the water filtrate is returned 307to the wet scrubber unit 208. Flue gas enters the wet scrubber unit 208from the flue gas stream 201. Scrubbed or clean gas then exits to thestack 209 wherein it is released to the atmosphere 308. The solids 309,being limestone and particulate matter from the flue gas stream such asfly ash, are separated from the waste sorption liquid and may be sold asa gypsum product. A sorbent composition that may be specially designedto disperse in the aqueous sorption liquid may, as is discussed above,enter the wet scrubber unit 208 with the flue gas stream 201.Alternatively, the sorbent composition may be fed into the wet scrubberunit 208 directly such as by mixing the sorbent composition with theother components in the slurry tank 304.

The solid sorbent (e.g., PAC), may remove mercury from the wet scrubberunit (i.e., from the aqueous sorption liquid) by adsorption. Thisrequires oxidation of the mercury, which drives the mercury into theliquid phase so that it can be captured by the PAC in the liquid. SincePAC has a high affinity for oxidized mercury and captures it in stableform in the pore structure, it is an ideal adsorbent.

FIG. 4 schematically illustrates a PAC mercury adsorption process in awet scrubber unit. In a wet scrubber unit, mercury may stay entrained inthe flue gas 401, be in solution 402 or be bound to solids 403, mainlyPAC. In the flue gas 401 mercury exists as a combination of elemental(Hg⁰), oxidized (Hg⁺²) and particulate phase mercury (Hg^(B)), i.e.,elemental and/or oxidized mercury associated with a solid particleentrained in the flue gas. Because the flue gas may have low nativeoxidation, the predominant form is naturally elemental, gaseous mercury(Hg⁰). Typically, only small amounts are oxidized (Hg⁺²) or inparticulate form with no mercury control reagents. When the flue gas isscrubbed with calcium carbonate and water 402, a mixture of mercuryspecies may be present in the gas/liquid interface and in theliquid/solid sorbent interface. The mercury molecules transport throughthe phases within the scrubber. An oxidant may be used to oxidize theelemental species to oxidized species which are easily bound by the PAC403. The PAC is then removed from the system with the slurry solids 309.Elemental mercury formed in the slurry liquid 307 may be recycled backto the scrubber or may diffuse back into the gas phase and ultimatelywill be re-emitted back into the flue gas 401 and escape through thestack 209. With a sorbent composition that is able to be well dispersedin the aqueous liquid phase of the wet scrubber unit, improved mercurycapture is possible.

The median particle size of a particle size distribution (PSD) iscommonly referred to as the D50. When particles of a given sizedistribution are dispersed into a liquid phase such as water, theprimary particles comprising the distribution may exhibit agglomerationwherein primary particles associate with each other leading to theformation of larger aggregates. The formation of aggregates duringparticle dispersion typically promotes faster gravity settling andconcomitant reduction in dispersion efficiency. Analysis of the particlesize distribution measured directly on the liquid phase dispersion ofthe particles will yield information pertaining to the extent of anyparticle agglomeration. Dispersive agents described herein may, forexample, reduce or even eliminate agglomeration of sorbent particles.Median particle size and PSD of the sorbent compositions can bemeasured, for example, by using a Micrometrics Saturn DigiSizer II(Micrometrics Instrument Corporation, Norcross, Ga.), which is ahigh-definition digital particle size analyzer. This instrument employsa light scattering analysis technique that utilizes advanced digitaldetection technology.

To assess the ability of sorbent compositions to disperse into anaqueous phase of a wet scrubber unit, wetting time of the sorbentcompositions can be measured in a wetting test. For this test, a givenamount of each sample sorbent composition is added to a predeterminedlevel of water (represented here by an aqueous sorption liquid samplefrom an operational wet scrubber unit). The time required for the samplesorbent composition to wet and completely enter the aqueous phase, i.e.,when substantially all of the sorbent composition leaves the aqueous/airinterface and fully enters the aqueous phase, becoming completelyimmersed, is observed visually and recorded. The test is performed atabout 20° C.

Utilizing such a test, it has been found that a sorbent composition asdisclosed herein (e.g., including an effective amount of a dispersiveagent) may enter an aqueous sorption liquid of a wet scrubber unit muchfaster than the untreated solid sorbent (i.e., the same solid sorbentwithout a dispersive agent). In one characterization, the time neededfor the sorbent composition to enter an aqueous sorption liquid is notgreater than about 90% of the time needed for the untreated solidsorbent to enter the aqueous sorption liquid, such as not greater thanabout 70% of the time needed for the untreated solid sorbent to enterthe aqueous sorption liquid, or even not greater than about 50% of thetime needed for the untreated solid sorbent to enter the aqueoussorption liquid.

To determine the stability of a dispersion of the sample sorbentcompositions, the time required for the sample sorbent compositions togravity settle from a fully homogenized dispersion can be measured in acomparative settling test. The greater the amount of time required forthe well-dispersed sample sorbent compositions to settle from theaqueous phase, the higher the stability of the dispersion and the moreeffective the sorbent composition for contacting and sequestering thecontaminant species.

The sorbent compositions disclosed herein may show increased ability toremove bromine/bromide from the aqueous sorption liquid of a wetscrubber unit as compared to untreated solid sorbents (i.e., without acapture agent). For example, the sorbent compositions of the presentdisclosure may have at least about a 50% increase, or even at leastabout a 60% increase, or even at least about a 70% increase, or even atleast about an 80% increase, or even at least about a 90% increase, oreven at least about a 95% increase in the ability to removebromine/bromide from the aqueous sorption liquid as compared tountreated solid sorbents.

EXAMPLES Example 1

Example 1 illustrates the ability of sample sorbent compositions toenter the aqueous sorption liquid of a wet scrubber unit by use of awetting test as described above. For this test, aqueous sorption liquidsamples from two separate industrial sites were taken, one being theaqueous phase of the scrubber slurry, obtained by filtering out thesolids, and the second being an aqueous sample from the effluent. Thecomparative Sample A sorbent composition is a prior art sorbent, namelyPowerPAC® (ADA Carbon Solutions, Littleton, Colo.), having no additionaltreatment or additive. PowerPAC® is an activated carbon sorbent with amedian particle size of about 25 micron, a fixed carbon content of about50 wt. %, mineral content of about 40 wt. %, and sum micropore andmesopore volume of 0.25 cc/g. Sample B is prepared by spraying anaqueous solution of tri-sodium phosphate, TSP, (Na₃PO₄) onto Sample A toyield a 0.1 wt. % concentration of TSP on the sorbent. To determine theease of wetting and dispersion of the samples, a 5 g aliquot of each ofSample A and B is added to 500 ml of the aqueous test solution at atemperature of 20° C. with no agitation.

For comparative Sample A, complete wetting or entry time into theaqueous liquid is about 90 min, whereas for Sample B complete wettingtime is about 10 min, indicating an 89% improvement in wetting time,i.e., the time needed for the Sample B sorbent composition to enter theaqueous sorption liquid is about 11% of the time needed for theuntreated solid sorbent to enter the aqueous sorption liquid. In arepeat test using an aqueous sorbent liquid from a different scrubberunit, Sample A wetting time is about 35 min, whereas Sample B wettingtime is about 9 min, indicating a 74% improvement in wetting time, i.e.,the time needed for the Sample B sorbent composition to enter theaqueous sorption liquid is about 26% of the time needed for theuntreated solid sorbent to enter the aqueous sorption liquid.

Example 2

The particle size distributions (PSD) of comparative Sample A (describedabove) and a Sample C are measured under dry conditions and upondispersion, and are compared to illustrate the degree of particleagglomeration upon aqueous dispersion. To create Sample C, a solution ofTSP (tri-sodium phosphate, Na₃PO₄) is sprayed onto comparative Sample Ato give a 5 wt. % concentration of TSP on the solid sorbent. FIG. 5Aillustrates PSD scan results for comparative Sample A under dry powderconditions, wherein the D50 is measured to be 21.7 micron. FIG. 5Billustrates PSD scan results for comparative Sample A followingdispersion in deionized water. The D50 of comparative Sample A isobserved to increase to 26 micron following aqueous dispersion, and asubstantial increase in the number of particles (i.e., aggregates) above100 micron in size is observed, indicating substantial particleagglomeration following aqueous dispersion.

FIG. 5C illustrates the aqueous phase PSD scan results from Sample C.Because Sample C was formed from Sample A, it has the same dry PSD of21.7 micron. The D50 of Sample C is measured to be 21.9 micron followingdispersion in water, which is very close to the D50 of the dry material.In addition, as seen in FIG. 5C, there are very few particles (i.e.,aggregates) greater than 100 micron in size in the aqueous dispersedSample C, indicating that the solid sorbent particles of Sample C do notagglomerate appreciably when dispersed in water and as such yield a morestable and effective dispersion.

Example 3

To test the stability of aqueous dispersions of the sample sorbentcompositions, a comparative settling test is performed as is describedabove. For this test, 5 g of each of comparative Sample A and Sample Cis poured onto the surface of 500 ml of an aqueous sorbent liquid from awet scrubber unit contained in a beaker. After each sample is completelywetted, both beakers are put in a jar tester. The mixing blade of thejar tester is set at 60 rpm to disperse the carbon in the test solutionfor 5 min. The blade is then stopped and the sample visually observed. Atimer is used to measure the time needed for the samples to settle onthe bottom. Comparative Sample A settles to the bottom in about 5 min,whereas Sample C settles to the bottom in about 9 min indicating betterdispersive qualities or suspension characteristics of Sample C in anaqueous environment.

Example 4

A beta test is a full-scale evaluation of a sample's efficacy at acoal-burning power plant. To test the effectiveness of mercury capturein a plant's wet scrubber unit, or wFGD, an example sorbent composition,Sample F, is used in an EGU. Sample F is created by spraying a TSPsolution onto Sample A above, to a dry TSP concentration of 0.1 wt. %.

This EGU is greater than 500 MW and fires coal. Referencing FIG. 2, theemission control system of the unit includes and is configured in thesequence of a boiler 201, hot-side ESP 204, air pre-heater (AH) 205, twowet scrubber units 208, and stack 209. The station burns a low-sulfurand high volatile content coal, with a heating value above 10,000Btu/lb. Standard ASTM methods are used to analyze a sample of the coal'sminerals and trace elements. The average dry sulfur content of the coalis 0.7 wt. % as measured by ASTM Method D 4239. Trace element analysisreveals that the coal contains less than 20 μg/g bromine (ASTM D4208-Br), 15-23 μg/g chlorine (ASTM D 6721), 1-5 μg/g arsenic (ASTM D3684/6357), 1-2 μg/g selenium (ASTM D 3684/6357), and 0.02-0.09 μg/gmercury (ASTM D 6722).

At this site, two wet scrubber units 208 are used per unit. Thescrubbers are open spray tower units, such as those that areschematically illustrated in FIG. 6. Each scrubber 600 is equipped witha primary vessel 601 and one primary dewatering system, which consistsof one bank of three hydrocyclones 602 with associated piping andvalves. The capacity of each primary vessel is about 900,000 gallons(gal), and the operating liquid volume is approximately 750,000 gal.Slurry 603 from the bleed slurry circuit is pumped to the primarydewatering hydrocyclone bank 602, and then the bleed slurry is directedto two dewatering cyclones. The primary cyclones separate the slurryinto two streams: overflow 604, or recirculated liquid, as in FIG. 3(307), with approximately 5% solid concentration, and underflow 605, orsolids, as in FIG. 3 (309), with approximately 50% solid concentration.The 5% slurry stream overflow 604 is routed to the hydrocyclone overflowtank and is pumped back to the primary vessel 601. The underflow stream605 (50% solid concentration) flows through the second dewatering systemand goes to belt filters where gypsum is separated.

The purpose of the beta tests is to demonstrate options for MATScompliance with the 1.2 lb/Tbtu mercury emission limit (30-day rollingaverage basis). In this example, the mercury in the boiler is oxidizedusing a calcium bromide (CaBr₂) coal additive and the Sample F sorbentcomposition is injected into the aqueous sorption liquid of a wetscrubber unit. The example sorbent composition is slurried as describedbelow and added into the wet scrubber unit recycle loop. Tests areconducted over a period of several months.

In tests, a solution of calcium bromide is applied continuously to thecoal. The Sample F sorbent composition is introduced to the aqueoussorption liquid of a wet scrubber unit at concentrations of 0 (i.e.,CaBr₂ addition to the coal only), a 1× addition of the Sample F sorbentcomposition to the absorber, and a 2× addition of the Sample F sorbentcomposition (i.e., approximately twice as much sorbent composition as inthe 1× example). The sorbent composition is added into the scrubberabsorber primary vessel after slurrying the sorbent composition inrecycled scrubber liquid. The sorbent composition is introducedintermittently by dosing the slurry into the wet scrubber unit vessel tomaintain the desired 1× or 2× sorbent composition dosage. This dosing ismanual, typically occurring once or twice a day, for a timed period ofinjection.

Mercury levels at the gas exit of the wFGD or the entrance to the stackare monitored using ASTM Method 30 B non-speciated traps. Traps aretypically changed out twice weekly, and sometimes more frequentlydepending on operational events. The operation of the wet scrubber unitincludes a continuous blowdown of slurry liquid and solids, recycle ofliquid overflow, sorbent composition addition intervals and continuousmake-up water. A concentration of Sample F sorbent composition isinitially dosed to a concentration higher than the desired concentrationin the absorber, but is depleted and made-up through the daily operationto reach a steady state. Full sorbent composition and Hg mass balancesare not obtained, but characterization of key streams is achieved.

In order to establish injection rates into the wet scrubber unit forboth initial dosing and maintenance of a consistent sorbent compositionlevel, a rough flow and mass rate balance is calculated around thescrubber. Concentrations of 1× and 2× of the Sample F sorbentcomposition are targeted in the wet scrubber unit for an initial dosageof the sorbent composition, and the addition of the sorbent compositionis then done in a daily dose that is timed by an operator of theinjection system. The manual addition of the sorbent composition istimed rather than weighed, so the relative concentration levels areapproximate.

Samples of the first unit's vessel slurry are taken during variousconditions over an extended period. The slurry samples are analyzed formercury content and the results are illustrated in FIGS. 7A and 7B.FIGS. 7A and 7B show both the mercury content of wet scrubber unitsolids in two different absorbers, A and B, respectively. The wetscrubber unit solids generally include mainly calcium sulfate hydrate(synthetic gypsum). In all cases the mercury content in the liquid isvery low, indicating that all mercury was either in the solid or gaseousphase. The increase in mercury content in the scrubber solids frombaseline to CaBr₂ addition alone and then with addition of the sorbentcomposition to the scrubber is a reflection of mercury transporting fromthe flue gas to the liquid phase and then to the solids as seen in FIG.4. The solid phase mercury amount includes the capture by the Sample Fsorbent composition. Referring back to FIGS. 7A and 7B, at baselineconditions, represented by the solid line, with no reagents added to theabsorbers, solid phase mercury concentration measures approximately 5-10μg Hg/100 g slurry. Once the flue gas mercury is oxidized through theaddition of calcium bromide to the coal, represented by the dotted line,improved transport of the (oxidized) mercury into the liquid phase isenabled, and this level increases to about 15-20 μg Hg/100 g slurry,indicating an improvement in mercury control (the balance of mercurywill go out with the flue gas). The point represented by the asterisk isan extrapolated point. With addition of the Sample F sorbent compositionto the scrubber, represented by the dashed lines, the solids mercurylevel increases (about 22-32 μg Hg/100 g slurry). Addition of thecalcium bromide oxidant increases Hg associated with the solids, howeveraddition of Sample F sorbent may increase the Hg associated with thesolids by almost two fold, as seen in FIG. 7B.

The wet scrubber unit of the plant described above, is operated with acombination of CaBr₂ feed onto the coal and addition of Sample F sorbentcomposition into the absorber vessel for six months. The mercury levelsin the flue gas at the stack, once Sample F was introduced, are depictedon FIG. 8. Prior to introduction of Sample F, flue gas mercury (trap)levels of about 3.6 lb/Tbtu were measured. The 30-day rolling average oftotal stack mercury is shown by line 802, compared to the MATS limit of1.2 lb/Tbtu mercury shown by line 803. Over time the rolling averageamount of mercury 802 drops, increases, and drops again, however thisaverage remains consistently below the MATS limit of 1.2 lb/TBtu 803.Further, no unusual foaming issues were observed during the testingperiod. These tests indicated that the Sample F sorbent composition wasan effective sorbent composition, with good wetting and dispersioncharacteristics leading to consistent mercury capture meeting the MATSlimit of 1.2 lb/TBtu over time.

While various embodiments of the present disclosure have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present disclosure.

What is claimed is:
 1. A wet scrubber unit disposed in a flue gas train,the wet scrubber unit containing an aqueous sorption liquid, wherein theaqueous sorption liquid comprises: water; an alkaline compound dispersedin the water; a solid sorbent; and a dispersive agent, wherein the solidsorbent is dispersed throughout the aqueous sorption liquid.
 2. The wetscrubber unit recited in claim 1, wherein the alkaline compoundcomprises limestone.
 3. The wet scrubber unit recited in claim 1,wherein the solid sorbent comprises a carbonaceous sorbent.
 4. The wetscrubber unit recited in claim 3, wherein the carbonaceous sorbentcomprises activated carbon.
 5. The wet scrubber unit recited in claim 1,wherein the dispersive agent is selected from the group consisting ofdispersants, deflocculants, surfactants, wetting agents, coupling agentsand mixtures thereof.
 6. The wet scrubber unit recited in claim 5,wherein dispersive agent comprises a deflocculant.
 7. The wet scrubberunit recited in claim 6, wherein the dispersive agent comprises aphosphate salt.
 8. The wet scrubber unit recited in claim 7, wherein thephosphate salt comprises tri-sodium phosphate.
 9. The wet scrubber unitrecited in claim 1, wherein the dispersive agent is disposed on thesolid sorbent.
 10. The wet scrubber unit recited in claim 9, wherein thedispersive agent is coated on the solid sorbent.
 11. The wet scrubberunit recited in claim 1, wherein the aqueous sorption liquid comprises acapture agent.
 12. The wet scrubber unit recited in claim 11, whereinthe capture agent is selected from the group consisting of peroxides,persulfates, silver compounds and mixtures thereof.
 13. The wet scrubberunit recited in claim 12, wherein the capture agent comprises ammoniumpersulfate.
 14. The wet scrubber unit recited in claim 12, wherein thecapture agent comprises a silver compound, the silver compoundcomprising a silver salt.
 15. The wet scrubber unit recited in claim 14,wherein the silver salt comprises silver nitrate.